This invention relates to a method for scanning an original picture by light beam at a high speed in a picture reproducing machine.
In a conventional method, an original picture through which the light beam is passed, is mounted to a transparent glass or plastic cylinder, and a light source and a pickup means are arranged inside and outside the cylinder, respectively. During the rotation of the cylinder, picture signals are consecutively picked up from the original picture by the pickup means. This method is carried out in a color scanner for photographic plate-making or a facsimile.
In this method, however, when the cylinder is rotated at high speed, the original picture is often detached or floated off the cylinder by the centrifugal force, and thus the pickup of the picture signals becomes incorrect. Hence, the rotation speed of the cylinder is restricted, and thus the operation speed of the machine is restricted, which means the difficulty of raising the operational speed.
In order to overcome this disadvantage, a plane light scanning method has been developed. In this method, the original picture mounted to the flat plane is scanned along a straight line by means of a light spot condensed by a combination of a light deflector such as a galvanometric mirror and a rotary polygonal mirror, and an optical system for focusing a picture image while the light beam reflected from or passed through the light spot is picked up by a photoelectric element of the pickup means.
In this method, however, when the light beam is separated into color separated light beams for primary colors, in order to receive each of the color separated light beams by means of each photoelectric element, a complicated optical system is required, and in particular, it is difficult to obtain an unsharp signal. Further, the accuracy of the picture image, or the size of the picture element depends on the diameter of the light spot focused, but, in this case, it is difficult to obtain an equal diameter of the light beam through the entire scanning line.
In order to prevent the picture image from being distorted, a f.multidot..theta. lens which moves the scanning light spot in proportion to the deflection angle of the deflector, has been used for the light scanning. Now, only the f.multidot..theta. lens having a narrow range of deflection angles and a large diameter of the scanning light spot, is realized. Further, in such a f.multidot..theta. lens a single color light should be used. Accordingly, it is quite difficult to design a f.multidot..theta. lens which is capable of obtaining an equal light spot by using the white light which is to be separated into the primary colors for the photographic plate-making.
When red, blue and green single color lights are used instead of the white light for the color separation scanning operation, the construction of the optical system becomes very complicated, and if laser tubes are used for generating the scanning lights, it requires quite high cost, which is uneconomical. Further, in order to compensate the inpurity component of the coloring material of the color film, a masking signal should be input depending on the kind of the color film, and since the compensation value depends on the kind of the color film, a complicated input signal is required. Thus, in practice, this method is almost impossible to be realized.